drm/shmem-helper: Use refcount_t for pages_use_count

drm/shmem-helper: Use refcount_t for pages_use_count

Use atomic refcount_t helper for pages_use_count to optimize pin/unpin
functions by skipping reservation locking while GEM's pin refcount > 1.

Acked-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Suggested-by: Boris Brezillon <boris.brezillon@collabora.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.d>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250322212608.40511-9-dmitry.osipenko@collabora.com